This invention relates to apparatus for accurately and continuously measuring temperature of a target material regardless of the emissivity of the material being measured.
One of the difficulties in measuring the temperature of a target material is the fact that such measurement is completely at the mercy of changes in emissivity of the surface because the amount of radiation from a surface at a particular temperature varies with the emissivity. The emissivity of a target material frequently changes in an unpredictable manner because of physical differences in different areas of the target which could be caused by changes in manufacturing operations, changes in the surfaces viewed due to oxidation or to the presence of foreign materials such as dirt, oils, waxes and the like which have emissivities that differ from that of the underlying target material. The perfect radiator or black body is characterized by the fact that the energy which it emits depends only on the temperature of the body. A non-black body radiator emits only a fraction of the energy emitted by a perfect radiator. This fraction is known as the emissivity of the body. Accordingly, in order for the energy emitted by the non-black body to be accurately determined, the emissivity must be known. However, there are many target materials whose temperature is desired to be known, such as hot rolling of tin plate, processing of plastic films at elevated temperatures and the like which may involve variations in the surface for example, by oxidation or other means which change the emissivity of the surface whose temperature is being measured. As a result the temperature measuring device will provide an inaccurate reading.
A number of approaches have been utilized to compensate or treat this problem. For example, in U.S. Pat. No 2,611,541 to Gray a heated source whose temperature is controlled is spaced from a workpiece whose temperature is to be measured, where the function of the source is to provide a known reflective background which reduces emissivity errors but does not eliminate them. Gray provides a pyrometer which views only the workpiece directly and gives a signal representative only of the temperature of the workpiece. The heated source of Gray is independent of the actual temperature of the workpiece and accordingly, a change in emissivity of the workpiece will still cause an error in the pyrometer output signal which is used to measure the temperature.
In Weiss U.S. Pat. No. 3,285,069, which is assigned to the Assignee of the present invention, the problem is treated using a differential infrared detector embedded in a temperature control plate which is mounted close to the target. One of the detector elements of the differential detector responds to the plate temperature and the other detector element to the target temperature. The differential detector signal is employed as an error signal to a temperature controller which drives the reference temperature toward that of the target. When the error signal is zero, the plate will be at the same temperature as the target regardless of the emissivity of the target material. This plate temperature is read out which in effect is the target temperature regardless of the emissivity of the target material.
The problem of this approach is that the differential detector is embedded in the reference plate, and accordingly must operate at the target temperature. The target temperature is often too high for the detector to withstand, and therefor the detector is destroyed. Another problem is that the plate must be relatively massive to contain the detector which restricts the frequency response. The more massive the reference plate the more difficult it is to rapidly heat the plate to elevated temperatures.